Alkali metal chlorate, and especially sodium chlorate, is an important chemical in the cellulose industry where it is used as a raw material in the production of chlorine dioxide, which is an important bleaching chemical for cellulose fibres. Alkali metal chlorate is conventionally produced by electrolysis of alkali metal chlorides in open non-divided electrolytic cells provided with hydrogen evolving cathodes. The overall chemical reaction taking place in such cells is MeCl+3H2O→MeClO3+3H2, where Me is an alkali metal. This reaction has a cell voltage of 3 V.
In the past, also electrolytic cells provided with oxygen consuming gas diffusion electrodes have been attempted for the production of chlorate. The English language Chemical Abstract (AN 1994:421025) of Chinese Patent Application No. 1076226 discloses such cells for the preparation of sodium chlorate. The gas diffusion electrode reduces oxygen supplied to the gas chamber adjacent to the gas diffusion electrode. The reduction reaction taking place at the gas diffusion electrode (gas diffusion cathode) is 2H2O+O2+4e−→4OH−. The oxidation reaction taking place at the anode is 2Cl−→Cl2+2e−. The cell voltage for the overall chemical reaction in the gas diffusion electrode cell is about 2 V, which implies that considerable operation costs can be saved by replacing the above described hydrogen evolving cathode with a gas diffusion electrode acting as cathode.
Operation of a cell as disclosed in English language abstract of Chinese Patent Application No.1076226, however, will instantly lead to poisoning of the gas diffusion electrode since the reaction products HClO , ClO−, and ClO3− formed at the anode will diffuse freely in the electrolyte and undesired side reactions will inevitably take place at the gas diffusion electrode according to the formulas below:HClO+2e−→Cl−+OH−  (1)ClO−+H2O+2e−→Cl−+2OH−  (2)ClO3−+3H2O+6e−→Cl−+6OH−  (3)
In many alkali metal chlorate processes, alkali metal chromates are employed to suppress reactions 1-3. However, alkali metal chromates can also have a negative impact on the gas diffusion electrode which quickly will deactivate upon contact with the chromate ions.
Production of chlorate may require considerable amounts of hydrochloric acid and alkali metal hydroxide, which also implies a considerable cost. Furthermore, the handling of these chemicals is complicated because of the rigorous safety requirements involved in transportation, storage and dosage.
An object of the present invention is to overcome the problems referred to above and at the same time provide an energy-efficient electrolytic process for the production of alkali metal chlorate. A further object of the invention is to provide a process which makes a large portion of externally added pH-adjusting chemicals superfluous to the process.